The disclosure relates generally to turbine airfoil cooling, and more particularly, to two portion cooling passages for a turbine airfoil.
The airfoils of gas turbine blades and nozzles are exposed to excessive heat loads. Airfoils are typically covered with a high concentration of a thermal barrier coating (TBC). Consequently, the TBC experiences spalls, which makes cooling the airfoils more difficult. In order to cool the airfoils, a coolant is typically introduced through cooling passages from an interior chamber of the airfoil through holes to an exterior surface of the airfoil. The cooling passages are arranged in large numbers, which creates many holes in the airfoil. At the leading edge, cooling hole arrangements may be referred to as a showerhead arrangement. Ideally, the coolant creates a cooling film, i.e., a flow across and close to the surface of the airfoil, which extends downstream along a surface of the airfoil.
On the leading edge, cooling passages with traditional round or conical shaped exit holes are radially oriented relative to the surface, i.e., they are drilled perpendicularly relative to the hot gas flow direction. Consequently, the cooling flow has to make a sharp turn and is susceptible to blowing off of the airfoil surface, which may reduce the coolant coverage and laterally-averaged cooling effectiveness. Cooling passages having shaped diffusion exit holes are typically used in other regions on the airfoil and have a relatively high cooling effectiveness, but have not been successfully used in the leading edge because of the small radius of curvature of the leading edge. That is, the shaped diffusion exit holes need to be drilled nearly perpendicular to the surface using traditional manufacturing methods. This arrangement results in reduced cooling performance.
In addition to the above challenges, there is an increased need to reduce overall coolant usage to meet forward lean efficiencies while also maintaining film cooling at the leading edge. In order to reduce overall cooling flows, fewer cooling passages are typically used, resulting in increased spacing between holes. Each cooling passage must therefore have increased laterally-averaged cooling effectiveness and increased lateral coolant coverage. Many advanced film designs rely on additive manufacturing to create improved cooling passages, but there are many challenges that need to be overcome before this technology can be put into widespread use in the hot gas path section.